Differential protection device including a rectifier and a toroid having a nanocrystalline core

ABSTRACT

A differential protection device includes a current transformer comprising a toroid formed by a magnetic core, primary windings formed by the active conductors of the installation and a secondary winding at the terminals of which a differential fault signal is established when a differential fault occurs in the primary windings, a tripping relay of a current breaking apparatus, and a secondary connection circuit connecting the relay to the terminals of the secondary winding, the secondary connection circuit being arranged to bring about tripping when the signal exceeds a preset threshold. The above-mentioned secondary circuit comprises a full-wave rectifier and a capacitor, connected in parallel with the secondary winding of the toroid, and the magnetic core is made of a nanocrystalline material.

BACKGROUND OF THE INVENTION

The present invention relates to a protection device of an electricalinstallation in which a tripping relay is activated when a fault signalin a differential transformer exceeds a preset threshold.

BRIEF DESCRIPTION OF THE PRIOR ART

Trip devices by fault current have been used for many years forprotection of machines and people. For protection of people, thetripping current can be about 30 mA, whereas it is in a range of about300 to 500 mA for protection of machines.

However, during the last few years, more and more electronic deviceswith current rectifier effect have been incorporated in numerouselectrical apparatuses. These rectifier effects may generate a DCcomponent liable to influence the operation of the differential device.The increased use of electronic trip devices, notably in householdappliances, also requires the latter to respond in complete safety notonly to alternating currents but also to pulsed DC fault currents. Thelimit values defined for trip devices of this kind have been set by thestandard VDE 0664. Trip devices of the previously mentioned kind areknown, which meet this specific requirement, wherein the magnetic coreof the transformer toroid is made of a crystalline material designed forthis use. The main quantities characteristic of these materials are theinduction amplitude B for a sinusoidal excitation current, the staticinduction elevation ΔB stat for a half-wave rectified sinusoidalexcitation current, and the dynamic induction elevation ΔB dyn for afull-wave rectified sinusoidal excitation current.

In these trip devices, it is known to fit a capacitor between thetransformer secondary winding and the relay tripping winding in order toincrease the sensitivity of the trip device by increasing the power atthe level of the relay. An oscillating circuit is thus formed by thesecondary winding and the capacitor. The resonance frequency of thisoscillating circuit then has to be tuned with the frequency of thevoltage in the secondary winding due to the fault current. Tuning ofthis resonant circuit is performed by defining the number of turns ofthe secondary winding based on the capacitance values prescribed for thecapacitor, and the tripping conditions of the trip device aredetermined. However, the number of turns finally set simply represents acompromise between the different forms of fault current.

European Patent application EP-0,563,606 describes a current transformerfor a trip device, enabling safe interruption of a user circuitsubjected to pulsed currents to be obtained, tripping being performed ina manner practically independent from the form of the fault current.These results are obtained due to the use of a magnetic core made of ananocrystalline material presenting the following magneticcharacteristics: Br/Bs<0.3, ΔBdyn>0.6T, for a field intensity amplitudeof 100mA/cm, ΔBdyn max>0.7T and ΔBdyn/B>0.7, these magnetic cores havingbeen achieved in two stages.

The magnetic quantities presented in this patent are in fact interestingfor a trip device setup using a capacitor. The presence of thiscapacitor and the fact of obtaining ΔBdyn/B>0.7 enable a moresymmetrical current form to be obtained on the secondary. In this case,the polarized electromagnetic trip device operates for two moresymmetrical thresholds, which results in a greater ease of settingadjustment when manufacturing. However, although they are advantageous,these features are nevertheless constraining, as they require aparticular treatment of the magnetic core materials.

Furthermore, in this patent, the trip device does not enable anefficient differential protection to be obtained for common applicationssuch as graduators which can have very brief opening angles ranging from135° to 180°.

SUMMARY OF THE INVENTION

The present invention overcomes these problems and proposes adifferential protection device whose sensitivity to pulsed currents isimproved without constraining magnetic characteristics or the magneticcore of the transformer toroid. The invention, in a particularembodiment, in addition enables a wider protection to be achieved, inpulsed currents, notably for angles α>135° concerning certaingraduators.

For this purpose, the object of the present invention is to achieve adifferential protection device of the kind previously mentioned, thisdevice being characterized in that the above-mentioned secondary circuitcomprises a rectifier and that the magnetic core of the toroid is formedby a nanocrystalline material.

According to a particular embodiment of the invention, the magnetic coreis made from a soft magnetic iron based alloy made up of at least 50% offine crystal grains of a size smaller than 100 nm and containing, inaddition to an iron content greater than 60% of iron atoms, 0.5 to 2% ofcopper, 2 to 5% at least of one of the following metals, niobium,tungsten, tantalum, zirconium, hafnium, titanium and/or molybdenum, 5 to14% of boron and 14 to 17% of silicon.

According to a particular embodiment, the above-mentioned rectifiercomprises two diodes respectively connecting the two ends of thesecondary winding to one of the ends of the coil of the relay whereasthe other end of the coil is connected to a mid-point of the secondarywinding.

Advantageously, the two diodes are Zener diodes.

According to another embodiment, the above-mentioned rectifier comprisesa diode bridge connecting the two ends of the secondary winding to theterminals of the coil of the relay.

According to a particular feature, the secondary circuit comprises inaddition a capacitor connected in parallel with the secondary winding ofthe toroid.

According to another feature, the voltage surge factor of the secondarycircuit is about 3.5.

According to another feature, the resonance frequency of the circuitcomprising the secondary winding of the toroid and the capacitor isabout 120 Hz.

According to a particular feature, the secondary circuit comprises inaddition a storage capacitor and a device for comparison connected tothe storage capacitor and comprising a monitoring output connected tothe control device of the relay, so as to supply a tripping signal tothe relay if the value of the voltage of the capacitor is greater than apreset threshold.

Advantageously, the comparison device comprise a comparator or a voltagethreshold diode.

Preferably, the control device of the relay comprise a thyristor.

BRIEF DESCRIPTION OF THE FIGURES

But other advantages and features of the invention will become moreclearly apparent from the following description, referring to theaccompanying drawings given as examples only and in which:

FIG. 1 is a curve representative of the variation of the trippingthresholds for a crystalline material and a nanocrystalline material.

FIG. 2 illustrates a first embodiment of a trip device according to theinvention;

FIG. 3 illustrates a second embodiment of a trip device according to theinvention;

FIG. 4 illustrates two curves representing on the y-axis the voltagesurge factor and on the x-axis the ratio C/C(50 Hz), respectively for acrystalline material and a nanocrystalline material.

FIG. 5 illustrates the relative reduction of the thresholds in class A,achieved by the nanocrystalline compared to the crystalline according tothe opening angle.

FIGS. 6a and 6b illustrate a third embodiment of a trip device accordingto the invention.

DETAILED DESCRIPTION

In FIGS. 2, 3 and 6a and 6b, three embodiments can be seen respectivelyof a differential trip device D according to the invention designed tobe incorporated in or associated for example to an electrical circuitbreaker (not shown) for breaking the active conductors supplying anelectrical installation. This device D comprises commonly to the threeembodiments, a differential transformer 1 formed by a magnetic coretoroid 2 comprising a primary winding 20 formed by the active conductorsof the installation passing through the toroid 2, and a secondarywinding 3 connected to the coil of a tripping relay 4 of the polarizedtype, by a secondary connecting circuit 5. The magnetic core of thetoroid 2 is made of nanocrystalline material.

The secondary circuit 5 of the trip device D represented in FIG. 2 isformed by two diodes 6, 7 connected on input respectively to the twoends of the secondary winding 3 and on output to the positive pole ofthe tripping relay 4, whereas the negative pole of the relay 4 isconnected to a mid-point 3a of the secondary winding 3 of the toroid 2.In the embodiment illustrated in FIG. 3, the secondary circuit 5comprises in addition a capacitor 8 connected in parallel to thesecondary winding 3 of the toroid 2. It should be noted that zenerdiodes or other equivalent devices will be advantageously used toprevent spurious tripping.

In the embodiment illustrated in FIG. 6a, the secondary circuit 5comprises a tuning capacitor 13 connected in parallel with the secondarywinding 3 of the toroid 2 and with a rectifier bridge P comprisingelements 9-12 whose outputs are connected in parallel to a storagecapacitor 14, which is connected in parallel with the relay 4 and athyristor 15 mounted in series, and with a threshold circuit comprisinga control output connected to the thyristor 15. The threshold circuitmay comprise a comparator 16 or a voltage threshold diode 20 as shown inFIG. 6b.

For the first two embodiments of FIGS. 2 and 3, the tripping relay 4commands tripping of the circuit breaker, when a fault current in theprimary windings, exceeds a preset tripping threshold. When the voltageobtained on the secondary winding 3 does not present two symmetricalhalf-waves, the rectifier 6, 7 adjusts the current on the secondarywinding, necessary for the polarized relay 4 to operate for twosymmetrical thresholds.

In the third embodiment of FIGS. 6a and 6b, the threshold circuitsupplies a tripping signal to the relay 14 via the thyristor 15, whenthe voltage at the terminals of the storage capacitor 14 exceeds acertain threshold.

In the embodiment illustrated in FIG. 2, operation of the trip device Dwith a good sensitivity to pulsed currents can be obtained for exampleby using a nanocrystalline material presenting the following magneticcharacteristics: ΔBdyn(100mA t/cm)<0.6T, ΔB dyn max<0.7T andΔBdyn/B<0.7.

In FIG. 1, curve (a) represents the standard boundary establishing thetripping thresholds, in terms of the opening angle, according to thestandard VDE 0684. Curve (b) represents the variation of the thresholdsin terms of (α) when a crystalline material (comprising 78% NI) is usedfor the magnetic core of the toroid 2 in a trip device (D) whosesecondary circuit comprises a rectifier, whereas curve (c) representsthe same variation for use of a nanocrystalline material in the sametype of trip device.

Referring to FIG. 1, it can be seen that the use of a nanocrystallinematerial for the magnetic core of the toroid 2 in a secondary circuit 5with rectifier enables differential tripping thresholds (S) to beobtained (curve c) in terms of the opening angle (α) which areappreciably lower than those established by the standard boundary (curvea), for opening angles up to 135°.

The reduced losses proper to nanocrystallines materials able to behighlighted by measurement of ΔB dyn in terms of the frequency enable adifferential protection to be obtained with a widened opening angle (α),as illustrated in FIG. 5. This curve represents on the y-axis thereduction of the thresholds (R) in %, in pulsed currents, achieved byuse of nanocrystalline material compared with a crystalline materialcomprising 55% NI.

In the embodiments illustrated in FIGS. 3 and 6a and 6b, the capacitor 8enables the power supplied to the relay 4 to be increased. The use ofthe nanocrystalline material enables sufficient energy to trip the relay4 to be picked up and transmitted to the secondary winding 3. Thesecondary winding 3 and capacitor 8 form a resonant circuit whoseresonance frequency is chosen in such a way that the voltage surgecreated by the capacitor 8 is about 3.5 times greater than thatoccurring without a capacitor. It can be noted that the voltage surgefactor (f) is defined as being the ratio between the voltage of thecapacitor 8 at a given frequency over the voltage at the terminals ofthe secondary winding 3, without a capacitor, for the same frequency.The resonance frequency is also chosen so that the pass-band of thefilter comprising the capacitor does not chop the harmonics of frequencyhigher than 50 Hz present in the pulsed current signals too much,notably for α=135°. When a crystalline material is used, this frequencyis in general about 75 Hz.

When the material constituting the magnetic core is a nanocrystallinematerial, due to its low losses it generates a voltage surge twice thatof a traditional crystalline material, as can be seen in FIG. 4. Curvesd, e of this FIG. 4 represent on the y-axis the voltage surge factor f,and on the x-axis the ratio between the capacitance c of the capacitorand a capacitance value for a resonance of 50 Hz (C, 50 Hz),respectively for a nanocrystalline material (d) and for a traditionalcrystalline material (e). It can thus be seen on these curves that for avoltage surge coefficient value of 3.5, the corresponding quantity C/C(50 Hz) for a nanocrystalline B is lower than that A of the crystalline,which enables a resonance frequency of about 120 Hz to be chosen. Thisresults in a widening of the differential pass-band, which enables awider protection range to be achieved in class A (pulsed currents), i.e.for angles <135°α<180° (these angles being present in certaingraduators).

It can be noted that the magnetic core could advantageously be formed bya soft magnetic iron based alloy made up of at least 50% of crystallitesof a size smaller than 100 nm and containing the following atomicpercentage: in addition to an iron content greater than 60%, 0.5 to 2%of copper, 2 to 5% at least of one of the following metals, niobium,tungsten, tantalum, zirconium, hafnium, titanium and/or molybdenum, 5 to14% of boron and 14 to 17% of silicon.

A differential protection device has thus been achieved enablinglowering of the pulsed current tripping thresholds to be achievednotably for opening angles α>135°, due on the one hand to the low lossescharacterizing nanocrystalline material and on the other hand to awidening of the pass-band of the assembly formed by the toroid andtuning capacitor.

Naturally, the invention is not limited to the embodiments described andillustrated which have been given as examples only.

On the contrary, the invention also comprises all the technicalequivalents of the means described as well as their combinations ifthese are made according to the spirit of the invention.

What is claimed is:
 1. A differential protection device for anelectrical installation sensitive to pulsed currents, comprising(a) adifferential transformer including a toroid having a magnetic coreformed of nanocrystalline material, primary windings formed of theactive conductors of the installation, and a secondary winding in whicha fault signal is established when a differential fault occurs in saidprimary windings; (b) a tripping relay; and (c) means connecting saidtripping relay with said secondary winding, said connecting meansincluding a capacitor connected in parallel with said secondary windingto define a resonant circuit having a resonance frequency for generatingsaid fault signal as a voltage surge for said relay and rectifier meanswherein said rectifier means performs symmetrization of said pulsedcurrent which responds to said fault signal in said secondary windingfor activating said tripping relay, wherein said connecting meanscomprises in addition a storage capacitor and means for comparison, saidcomparison means being connected to the storage capacitor and comprisinga monitoring output connected to a control means of the relay to supplya tripping signal to the relay if the value of the voltage of thestorage capacitor is greater than a preset threshold.
 2. The deviceaccording to claim 1, wherein said magnetic core is a soft magnetic ironbased alloy, comprising more than 50% of fine crystal grains of a sizesmaller than 100 nm and, wherein the fine crystal grains comprise 63 to78.5% of iron atoms, 0.5 to 2% of copper, 2 to 5% of at least one of thefollowing metals, niobium, tungsten, tantalum, zirconium, hafnium,titanium, and/or molybdenum, 5 to 14% of boron and 14 to 17% of silicon;wherein the total quantity of these elements does not exceed 100%. 3.The device according to claim 1 wherein said rectifier comprises twodiodes (6,7,) respectively connecting the two ends of the secondarywinding (3) to one of the ends of said relay (4) whereas the other endof said relay (4) is connected to a mid-point (3a) of the secondarywinding (3).
 4. The device according to claim 3, wherein said two diodes(6,7) are two Zener diodes.
 5. The device according to claim 1 whereinsaid rectifier comprises a diode bridge connecting the terminals of thesecondary winding (3) to the terminals of the relay (4).
 6. The deviceaccording to claim 1, wherein a voltage surge factor (f) of theconnecting means (5) is about 3.5.
 7. The device according to claim 6,wherein the resonance frequency of the resonant circuit comprising thesecondary winding (3) of the toroid (2) and the parallel connectedcapacitor (8) is about 120 Hz.
 8. The trip device according to claim 1,wherein said comparison means (16) comprise a comparator.
 9. The tripdevice according to claim 1, wherein said comparison means (16) comprisea voltage threshold diode.
 10. The trip device according to claim 1,wherein said control means of the relay (4) comprise a thyristor (15).